Semiconductor based sensors and devices for detecting electromagnetic radiation are known in the art. Examples of such sensors are disclosed in EP-A-739039 and in W093/19489. These sensors are implemented in a semiconductor substrate in CMOS- or MOS-technology. In these sensors, the regions adapted for collecting charge carriers being generated by the radiation in the semiconductor substrate are forming a p-n or a n-p junction with the substrate that is of a n type conductivity or p type conductivity respectively. Such junctions are called collection junctions. Among the image sensors implemented in CMOS- or MOS-technology, CMOS or MOS image sensors with passive pixels and CMOS or MOS image sensors with active pixels are distinguished. The sensors of EP-A-739039 and W093/19489 are active pixel sensors.
An active pixel is configured with means integrated in the pixel to amplify the charge that is collected on the light sensitive element or component in the pixel. Passive pixels do not have said means and require a charge-sensitive amplifier that is not integrated in the pixel and is connected with a long line towards the pixel. Due to the additional electronics in the active pixel, an active pixel image sensor may be equipped to execute more elaborated functions, which can be advantageous for the performance of the imaging device or system based on the sensor. Said functions can include filtering, operation at higher speed or operation in more extreme illumination conditions. It remains however a main drawback of active pixel CMOS or MOS image sensors, and to a lesser extent also of passive pixel sensors, that a significant part of the surface of the pixel is used for readout circuitry.
It is known that the charge sensitive volume of a p-n or n-p junction is larger than the depletion layer of the junction. In fact all charges generated within a so called recombination length from the collection junction have a chance of diffusing to that junction and of being collected. Based on this mechanism it is possible to make a sensor with a small junction and yet a large photosensitive volume. Photosensors can be made with junctions of 3 by 2 micrometers and with a recombination length of 15 .mu.m. Thus such detector has an apparent front size or photosensitive region of 30 .mu.m diameter. However if a non-related electronic circuitry such as readout circuitry is placed in the neighborhood of such collection junction, part of the charges that otherwise would have reached the collection junction will be collected by junctions or components of the readout circuitry. The charge carriers generated by light falling on the regions of the detector that are used for readout circuitry therefore are mainly collected by the junctions of this readout circuitry. The area taken by the readout circuitry in the pixels therefore is lost for collecting the radiation and this is essentially the reason for the low fill factor or low sensitivity of active pixel based sensors.
The book "Solid-State Imaging with Charge Coupled Devices", of A. Theuwissen (Kluwer Academic publishers, 1995 ISBN 0-7923-3456-6) gives an overview of the present state of the art of semiconductor based imaging devices, such as CCDs and MOS cameras. Chapter 7 of this book is devoted to the topic of increasing the sensitivity or the effective fill factor of a pixel. It covers such methods as the use of micro lenses or the use of a photo conversion top layer.
In the article "XYW detector a smart two-dimensional particle detector," by Bart Dierickx, published in Nuclear Instruments and Methods in Physics Research, vol. A275 (1989), p.542-544, FIG. 4 shows 4 methods to fabricate semiconductor based pixel devices. The first method is based on classic CMOS processing. The disadvantage of this method is quoted to be a low fill factor, since part of the semiconductor surface is occupied by readout electronic circuitry. The other methods attempt to solve the fill factor problem by using non-standard-CMOS techniques: flip-chip (bump bonding), SOI (silicon-on-insulator) technology, or moving the photon-sensitive zone to an amorphous top-layer.
In the article "Design and Performance of semiconductor detectors with integrated amplification and charge storage capability," by P. Klein & al, published in Nuclear Instruments and Methods in Physics Research, vol. A305 (1991) pp. 517-526, FIG. 2 shows a semiconductor based sensor with a transistor junction wherein the charges being generated in the semiconductor substrate are prevented from diffusing into the source and drain junctions of the transistor by a junction-based barrier and are constrained to the gate, the gate being in contact with the source and drain.
Avalanche photo diodes (ADPs) are also known in the art. An APD can consist of a structure having layers in a sequence p.sup.++ /p.sup.- /p.sup.+ /n.sup.++, where the p++ layer is the backside contact, the p- layer is a detection layer, the p+ layer is serving for avalanche multiplication of electrons, and the n++ layer is finally the collection layer of the multiplicated electrons. The dopant level and width of the multiplication layer is very critical. Furthermore the applied voltage over such a structure is rather high, close to the electric breakdown of the structure. The p-/p+ layer transition is not acting as a barrier for the diffusion of electrons, but as an avalanche multiplication device.
Aim of the Invention
It is an aim of the present invention to disclose a semiconductor based device for detecting electromagnetic radiation wherein substantially all of the charge carriers being generated by said radiation in the semiconductor are collected into the collection junctions and regions, instead of into regions and junctions of the readout circuitry.
Summary of the Invention
The present invention relates first to a detector for electromagnetic radiation which includes a semiconductor substrate, said substrate comprises a first region and a second region, said first region and said second region being adapted for collecting charge carriers being generated by said radiation in said substrate. Said substrate further comprises a third region forming a barrier for substantially impeding the diffusion of said charge carriers to said second region.
In between the substrate and the first region, there is no barrier present or a substantially zero barrier present or a substantially lower barrier than in between said second region and said substrate.
Said third region can also form a substantially lower barrier for the diffusion of said charge carriers to said first region.
More particularly, the present invention relates to a semiconductor based detector for radiation with a small but effective barrier between the radiation sensitive volume in the semiconductor substrate and the regions and junctions with readout circuitry, and with no or a lower barrier between the radiation sensitive volume in the semiconductor and the regions and junctions adapted and meant for collecting the charge carriers being generated by the radiation.
According to a first aspect of the present invention, the detector can be such that at least part of the charge carriers that are generated in said substrate adjacent to, and preferably underneath, said second region are collected by said first region. Said substrate can have dopants of a first conductivity type, said first region and said second region having dopants of the other and second conductivity type, said third region having dopants of said first conductivity type, the doping level of said third region being higher than the doping level in said substrate.
According to another aspect of the present invention, said first region forms a junction with said substrate, and said second region forms a junction with said third region.
According to a further aspect of the present invention, a MOS based pixel structure for detecting light comprising the detector of the present invention is disclosed. In this case, said third region forms an electrostatic barrier for substantially impeding the diffusion of electrons or holes to said second region. Said electrostatic barrier can be formed at the interface between said third region and said substrate.
A substantially lower electrostatic barrier for the diffusion of electrons or holes from said substrate to said first region is formed at the interface between said first region and the substrate. Said second region can be part of a circuit for processing the signal being generated by said charge carriers in said first region. This signal can be the current being generated in said first region. The pixel structure can further comprise a fourth region adjoining said first region and said substrate, the substantially lower barrier then being formed at the interface between said fourth region and the substrate.
In other aspects of the present invention a method of manufacturing and a method of operating a CMOS based pixel structure for detecting electromagnetic radiation are disclosed.
The present invention further comprises the aspects of design integration of the principle of the invention in CMOS based pixel structures and sensors and in CCD's or CCD-like structures. The present invention advantageously can be applied in sensors for low-noise light detection applications.